Closed cycle mixed refrigerant systems

ABSTRACT

The present invention provides for a process for providing cooling and controlling the refrigeration in a cooling loop used in the production of liquefied natural gas. A cooling loop in contact with a heat exchanger contains a refrigerant composition and by controlling the amount of a component in the refrigerant composition, the necessary level of cooling provided to the heat exchanger can be maintained.

This application claims priority from U.S. Provisional PatentApplication Ser. No. 60/919,998 filed Mar. 26, 2007.

BACKGROUND OF THE INVENTION

The present invention provides for a method to continuously monitor andcontrol the refrigerant composition in a closed cycle mixed refrigerantsystem.

Liquefied natural gas (LNG) is produced by both small scale and largescale plants. The overall heating value of the LNG can be affected bythe percentage of heavier hydrocarbon components found in the naturalgas and hence, heavier hydrocarbons may need be removed prior toliquefaction. Some natural gases will also require removal of the heavyends to prevent operating problems in the liquefaction cycle

U.S. Pat. No. 6,530,240 B1 teaches a method for controlling a mixedrefrigerant based, natural gas liquefier system that utilizes anexchange of system refrigerant between the system and an externalstorage tank whereby the use of extremely high pressures in thecompressor discharge that is employed in conventional systems iscircumvented. This reference is directed to the control of mixedrefrigerant based natural gas liquefiers using low cost HVAC componentsdue to the risk of exceeding the pressure and temperature requirementsof the HVAC components. The pressure limitations are avoided byadjusting the pressure in the refrigerant circulation circuit to belowabout 175 psig by exchange of refrigerant with the refrigerant storagecircuit.

MRC Cycle for LNG/Air Separation Saves Money, D. T. Linnett, TenthAustralian Chemical Engineering Conference, 1982, Sydney, 24-26 Augustteaches alternative cycles for liquefaction of natural gas and theadvantages of mixed refrigerant cascade cycle. The MRC cycle uses asingle multi-component refrigerant comprising five components, nitrogen,methane, ethylene, propane, and butane. The mixture composition is suchthat it condenses over a very wide temperature range. The mixture iscompressed in a single compressor and then partially condensed againstcooling water. The liquid is separated, sub cooled, expanded to a commonlow pressure, then evaporated and recycled to the compressor. Theuncondensed vapor is further cooled and further partially condensed andthe same procedure is repeated.

Closed cycle mixed refrigerant cycle (MRC) based liquefaction systemsare commonly used for the liquefaction of natural gas. These systemshave been identified to offer improved efficiency by way of lower powerconsumption compared to a single component nitrogen based system. An MRCsystem for natural gas liquefaction uses three or more components. Onesuch mixture specified by the owners of the '240 patent for their smallscale LNG system uses a five component refrigerant mixture consisting ofnitrogen, methane, ethane, iso-butane and iso-pentane. In this mixture,the iso-pentane is liquid at the warm end or ambient temperature. Anumber of factors such as ambient temperature, discharge pressure of thehigh pressure refrigerant mixture in the cycle, two phase flowdistribution in the heat exchanger, and refrigerant component solubilityin oil in an oil-flooded compressor system, affect the overall coolingefficiency of the closed cycle refrigeration system. When the coolingefficiency of a system is affected, the power consumption for theliquefaction of natural gas will also be affected.

FIG. 1 illustrates a prior mixed refrigerant system. Natural gas is fedto the main heat exchanger 10 through line 1 and product liquefied isrecovered via valve 3 through lines 2 and 4. A mixed refrigerant offixed composition will enter the suction of a compressor as vapor only.The compressed refrigerant is fed through line 6 and is cooled in anevaporative cooler 20 and partially condensed. The two-phase highpressure mixture is then fed through line 7 into the main plate fin heatexchanger 10 and completely condensed as it passes down to theJoule-Thompson valve. The high pressure liquid refrigerant is expandedto a pressure close to the compressor suction pressure and producescooling by flashing and phase change. This high pressure liquidrefrigerant is fed through line 11 into the heat exchanger 10. Thisgas-liquid refrigerant mixture is completely vaporized in the lowpressure passage of the main heat exchanger 10 and providesrefrigeration to liquefy the high pressure refrigerant and the naturalgas. The warm gaseous refrigerant leaving the exchanger is fed throughline 5 to the compressor 15 thereby completing the refrigerant cycle.

This is a closed refrigerant system, so the total holdup/inventory isconstant and is distributed between the gas and the liquid phase. Thisis true for all closed refrigerant cycles. The refrigerant in the gasphase is a function of the system pressure with the balance being in theliquid phase. The primary liquid holdup is at the cold end of the mainheat exchanger. This suggests that the system pressure will determinethe liquid holdup. In this case, the primary liquid holdup will be atthe cold end upstream of the Joule-Thomson valve. The refrigerantcomposition everywhere in the loop is the same.

The present invention addresses the inefficiencies of these earliersystem by exploiting the difference in the liquid and gas phasecompositions at different locations in the loop.

SUMMARY OF THE INVENTION

The present invention provides for a method of controlling the liquidinventory in a closed loop refrigeration system in order to increase thequantity or relative percentage of the heavy component, iso-pentane,which is a liquid at ambient conditions (all other components arevapor), in the refrigerant mixture so that the warm end heat exchange inthe refrigeration system heat exchanger is optimized. This will improveliquefaction efficiency.

In a first embodiment of the present invention there is disclosed amethod for providing refrigeration to a natural gas liquefaction processwherein a cooling loop containing a refrigerant composition is directedthrough a heat exchanger comprising controlling said refrigerantcomposition continuously by changing the liquid level in a warm endphase separator.

The refrigerant composition contains iso-pentane amongst othercomponents. The liquid level in the warm end phase separator decreasesas the heat transfer required of the heat exchanger increases. Theliquid level is controlled on-line typically by a PLC in communicationwith the warm end phase separator. As the liquid level in the warm endphase separator increases, the heat transfer required of the heatexchanger, which is in fluid communication therewith will decrease.

The cooling loop and the heat exchanger are also in fluid communicationand the control of the refrigerant composition is performed by changingthe amount of iso-pentane present therein.

In another embodiment of the present invention there is disclosed amethod for providing cooling to a process for producing liquefiednatural gas comprising the steps:

-   -   a) contacting a stream of liquefied natural gas with a heat        exchanger;    -   b) contacting the heat exchanger with a cooling loop containing        a refrigerant composition;    -   c) adjusting the composition of the refrigerant by controlling        the amount of a heavier condensable component in the refrigerant        composition in response to performance variations in the heat        exchanger; and    -   d) recovering liquefied natural gas.

In this embodiment, the heavier condensable component is iso-pentane.The performance variations in the heat exchanger are selected from thegroup consisting of a temperature increase in the heat exchanger and atemperature decrease in the heat exchanger. The amount of the heaviercondensable component in the refrigerant composition will increase inresponse to an increase in temperature in the heat exchanger.Accordingly the amount of the heavier condensable component in therefrigerant composition will decrease in response to a decrease intemperature in the heat exchanger.

In another embodiment of the present invention, there is disclosed amethod for providing refrigeration to a natural gas liquefaction processwherein a cooling loop comprising two warm end separators and a cold endseparator are in fluid connection with each other comprising controllingthe liquid level in the warm end separators.

In this embodiment, the cooling loop contacts a heat exchanger. Therefrigerant composition contains iso-pentane and the control of theliquid level is performed by changing the amount of iso-pentane presentin the refrigerant composition. The liquid level in the warm endseparators decreases as the heat transfer required of the heat exchangerincreases.

The liquid level is controlled on-line typically by a PLC incommunication with the warm end phase separator. A first warm endseparator is in fluid communication with a second warm end separator.The liquid level in the warm end separators increases as the heattransfer required of the heat exchanger decreases.

In another embodiment of the present invention there is disclosed amethod for providing refrigeration to a natural gas liquefaction processwherein a cooling loop comprising a warm end separator and a cold endseparator are in fluid connection with each other comprising the steps:

-   -   a) controlling the liquid level in the warm end separator;    -   b) removing vapor from the cold end separator;    -   c) heat exchanging the vapor with a separate liquid nitrogen        storage system; and    -   d) returning the liquid from the exchange of step c) to a heat        exchanger.

The controlling of the liquid level is performed on-line. The coolingloop will contact a heat exchanger and the liquid level in the warm endseparator is controlled by changing the amount of iso-pentane present inthe liquid. The liquid level in the warm end separators decreases as theheat transfer required of the heat exchanger increases.

In a further embodiment of the present invention, there is disclosed amethod for providing refrigeration to a natural gas liquefaction processwherein a cooling loop comprising a warm end separator and a cold endseparator are in fluid communication with each other comprisingcontrolling the liquid level in the warm end separator and controllingthe flow of natural gas from a separator to a heat exchanger.

The liquid level in the warm end separator is controlled by changing theamount of iso-pentane present in the liquid and can be performedon-line. The flow of natural gas from a separator to a heat exchanger iscontrolled by the temperature of the separator where a gas stream willenter the heat exchanger and a bottom liquid stream is directed from theseparator for reentry into the separator.

The bottom liquid stream comprises butane, propane and pentane. The gasstream from the separator after exiting the heat exchanger is directedinto the separator.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a liquefied production process with a heatexchanger.

FIG. 2 is a schematic of a liquefied production process where the liquidlevel in the warm end separator is continuously controlled.

FIG. 3 is a schematic of a liquefied production process where secondwarm end storage is provided at the suction side of the refrigerantcompressor.

FIG. 4 is a schematic of a liquefied production process where there isan additional cold end control by removing vapor from the cold endseparator.

FIG. 5 is removal of the heavy hydrocarbon contaminants in the naturalgas by an intermediate draw.

FIG. 6 is a graph showing LNG production for three cases with differentwarm end temperatures and adjusted refrigerant compositions.

FIG. 7 is a graph showing the heat exchanger temperature profile for thedesign case 1 in Table 1 and FIG. 6.

FIG. 8 is a graph showing the heat exchanger temperature profile withincreased warm end temperature for case 2 in Table 1 and FIG. 6.

FIG. 9 is a graph showing the heat exchanger temperature profile for thecase with the increased warm end temperature and adjusted refrigerantcomposition for case 3 in Table 1 and FIG. 6

DETAILED DESCRIPTION OF THE INVENTION

FIG. 2 shows a mixed refrigerant system employing the methods of thepresent invention. In this system, both warm 30 and cold end 35 phaseseparators are present. There are different refrigerant compositions ineach of the phase separators and at the bottom of the main heatexchanger upstream of the Joule-Thompson valve.

For purposes of FIGS. 2, 3, 4 and 5, common components compressor,evaporative cooler, main heat exchanger, natural gas feed and liquefiedproduct recovery have been designated with the same numbers throughout.

By changing the relative liquid holdup in the separators and the heatexchanger backup, the refrigerant composition in the loop can beadjusted. The liquid level in the warm end separator is continuouslycontrolled online in order to regulate the iso-pentane in thefive-component refrigerant mixture depending on various inputconditions. If a higher level of warm end heat transfer is required inthe main heat exchanger, a higher amount of higher boiling iso-pentaneis required in the refrigerant mixture and this is achieved bydecreasing the level in the warm end separator.

The increased heavy components such as iso-pentane in the refrigerantmixture can help to counter the impact due to an ambient temperatureincrease, a discharge pressure decrease or due to increased solubilityof iso-pentane in the compressor oil.

In FIG. 2, there is represented one aspect of the present invention.Natural gas is fed into line 1 and enters the main heat exchanger 10.The liquefied natural gas will leave the main heat exchanger throughline 2 where its flow will be controlled by valve 3 and recovered vialine 4.

The refrigerants will circulate through line 12 and enter the compressor15 and travel through line 13 to an evaporative cooler. The colder andcompressed gas stream of refrigerants will travel through line 14 to awarm end separator 30 where the warmed stream of refrigerants will leavethe warm end separator 30 through line 29 and enter the top of the mainheat exchanger 10. The now cooled stream of refrigerants will leave themain heat exchanger 10 through line 21 and enter a Joule-Thompson valve22 and travel through line 23 to a cold end separator 25. The gaseousportion from the cold end separator 25 will leave through line 24 andreenter the main heat exchanger 10. The cold bottoms from the cold endseparator 25 will leave either through valve 26 or valve 27 and line 28where they will reenter the main heat exchanger 10.

The cold end bottoms from the warm end separator 30 will be pumped outthrough pump 16 and line 15 as well as be withdrawn through line 17 tovalve 18. Opening of the valve 18 will allow the colder bottoms totravel through line 19 back to the main heat exchange 10.

In an alternative embodiment of the present invention, FIG. 3 shows asecond warm end storage being provided at the suction side of therefrigerant compressor and is connected to the discharge side warm endseparator. This allows for a greater change in the iso-pentane quantityin the refrigerant mixture entering the main heat exchanger by allowingthe back and forth transfer of liquid between the suction side separatorand the discharge side warm end separator.

In a further embodiment of the present invention, the primary control ofthe refrigerant composition is performed at the cold end. For smallchanges in refrigerant compositions, this is achieved by increasing orlowering the level in the cold end separator. When the level isincreased, typically the concentrations of the lighter components suchas nitrogen and methane in the five component refrigerant mixture isdecreased and heavy components such as iso-pentane and iso-butane areincreased. When the level is decreased, the concentrations of lightercomponents are increased. The continuous control of the cold endseparator level allows the heat exchanger performance to be maximizedwhen issues such as two phase maldistribution or varying refrigerantcompressor discharge pressures are encountered.

In FIG. 3, there is represented a further aspect of the presentinvention. Natural gas is fed into line 1 and enters the main heatexchanger 10. The liquefied natural gas will leave the main heatexchanger through line 2 where its flow will be controlled by valve 3and recovered via line 4.

Refrigerant will leave the main heat exchanger 10 through line 36 andtravel to a suction side separator 35. SP 35 provides additional valveto store liquid from SP 45. Normally there is no liquid in streamthrough line 36. The bottoms from the suction side separator will leavethrough line 38 and travel via pump 40 to line 41 or they can berecycled through valve 39 and line 37 back to the suction side separator35. If these bottoms are not recycled, they are transmitted via pump 42and valve 33 back through line 44A to the main heat exchanger 10.

The gas leaving the suction side separator 35 will leave through line35A to compressor 15 and evaporative cooler 20. The cooled andcompressed refrigerant stream will travel to the warm end separator 45through line 45A and through pump 42 where they can be transmittedthrough valve 44 and line 44A to the main heat exchanger 10 or throughvalve 44 and line 43 back to the warm end separator 45. The gas from thetop of the warm end separator 45 will leave through line 46 and reenterthe main heat exchanger 10.

Once they have traveled through the main heat exchanger 10, therefrigerant mixture will leave through line 31 and their flow will becontrolled by a Joule-Thompson valve 32. Flow control could also be asuction pressure control. The refrigerant mixture will flow through line33 to the cold end separator 40 where the liquid from the bottom willtravel through valve 34 and line 34A back to the main heat exchanger 10or be recycled through line 34B to the cold end separator 50. The gasfrom the cold end separator 50 will leave through line 36 and reenterthe bottom of the main heat exchanger 10.

In a further embodiment shown in FIG. 4, additional cold end control isachieved by removing vapor from the cold end separator and heatexchanging it with a separate liquid nitrogen storage system to liquefya portion or all of it and returning the liquid to the cold side of themain heat exchanger. This allows for significant changes in theconcentration of the lighter components such as nitrogen and methane inthe five component refrigerant mixture.

In FIG. 4, there is represented another aspect of the present invention.Natural gas is fed into line 1 and enters the main heat exchanger 10.The liquefied natural gas will leave the main heat exchanger throughline 2 where its flow will be controlled by valve 3 and recovered vialine 4.

The refrigerant stream from the main heat exchanger 10 will leavethrough line 78 and connect with a suction side separator 55. Thebottoms from the suction side separator 55 will leave through line 57and be pumped around through pump 60 where they will either reenter thesuction side separator through line 61, valve 58 and line 56 or bedirected via line 62 to pump 63, although typically there will be nobottoms. The suction side separator 55 will also utilize line 59 tocreate a connection with line 57 and pump 60 to recirculate as necessarythrough valve 58 some bottoms withdrawn from the suction side separator55. The refrigerant bottoms have passed through pump 63 and line 64 willtravel through valve 66 and line 69 to reenter the main heat exchanger10 at the top.

The gaseous refrigerant mixture from the top of the suction sideseparator will leave through line 55A through compressor 15 and line 13and travel through the evaporative cooler 20 and line 14 into the warmend separator 65. There the refrigerant stream will leave through thebottom and line 65A and be returned through pump 63, line 64, valve 66and line 69 to the top of the main heat exchanger 10. A portion of thisstream may travel via valve 66 into line 67 and be returned to the warmend separator 65.

The top from the warm end separator will leave through line 68 andreenter the main heat exchanger 10 at the top. Both refrigerant streamsthat leave the warm end separator 65 through either line 68 or 69 willbe recovered from the bottom of the main heat exchanger 10 through line71 where they will be drawn through a Joule-Thompson valve 72. Thisstream will travel through line 73 into the cold end separator 70. Thegaseous mixture from the cold end separator 70 will leave through line74 and enter line 79 where they will enter the liquid nitrogenrefrigerant buffer 75. This stream may also reenter the main heatexchanger 10 at the bottom. The cold ends of the cold end separator willtravel to valve 77 and either be circulated through line 76 back to thecold end separator 70 or travel through line 78 into the bottom of themain heat exchanger 10.

The refrigerant stream that has entered the liquid nitrogen cooledrefrigerant buffer 75 will be withdrawn through valve 85 and line 86 toreconnect with line 73 for reentry into the cold end separator 70.

The liquid nitrogen cooled refrigerant buffer 75 will use liquidnitrogen as the buffer and this enters through line 81 and valve 82 andwill travel through line 83 and out through line 80 after it hasabsorbed heat from the refrigerant stream. The liquid nitrogen may alsotravel through valve 82 and line 84 where it can connect with valve 85for either reentry into the liquid nitrogen cold refrigerant buffer 75or pass through line 86 and line 73 to the cold end separator 70.

FIG. 5 demonstrates another aspect of the present invention where thereis removal of pentane or hexane by regulating the temperature and thenatural gas bypass flow. Natural gas is fed into line 1 and enters themain heat exchanger 10. The liquefied natural gas will leave the mainheat exchanger through line 2 where its flow will be controlled by valve3 and recovered via line 4

The natural gas feed can also travel through valve 106 and line 108 to aseparator 105 where the top gaseous stream lean in the heavies willleave through line 109 and travel for entry into the main heat exchanger10. The bottom liquid stream enriched in heavy components such aspropane, butane and pentane when present in the natural gas feed willleave through valve 112 and line 113 to a boiler (not shown). They canalso be recycled through line 111 to the separator 105. The natural gasfeed may also be directed through valve 106 to lines 107 and 110 forentry into the separator 105.

The warm stream leaving the main heat exchanger 10 through line 96 willenter compressor 15 and through line 13 enter evaporative cooler 20. Thecooled and compressed refrigerant stream will enter the warm endseparator 95 through line 14. The top gaseous refrigerant stream fromthe warm end separator will leave via line 95A for entry into the mainheat exchanger 10. The bottom liquid stream will leave via pump 99 andwill travel to valve 100 where they may be recycle to the warm endseparator through line 98.

The bottoms from the warm end separator may also continue through valve100 and line 91 into the main heat exchanger 10. This stream havingpassed through the main heat exchanger 10 will leave via line 91A andpass through a Joule-Thompson valve 92 where it will enter the cold endseparator 90 through line 93. The bottoms from the cold end separator 90will travel through valve 94 and either be recirculated through line 94Ato the cold end separator 90 or travel through line 96 for reentry intothe main heat exchanger 10. The top gaseous stream from the cold endseparator 90 will travel through line 97 for entry back into the mainheat exchanger 10.

TABLE 1 Effect of warm end temperature and refrigerant composition onliquefier efficiency. 1 2 3 Tmax ° C. 25.5 35 35 x isopentane mol % 12.912.9 15.5 LNG Prod. kg/hr 745 589 695 Relative LNG % of 100% 79% 93%Production design Case 1 is the design case, case 2 is a simulation fora 9.5K higher warm end temperature for the same refrigerant composition,and case 3 is also for the higher warm end temperature, but with anadjusted refrigerant composition. Only the amount of isopentane wasincreased in order to remove a temperature pinch at the warm end of theheat exchanger. FIGS. 7 through 9 show the effect of the increased warmend temperature and the compensating effect of the compositionadjustment.

While this invention has been described with respect to particularembodiments thereof, it is apparent that numerous other forms andmodifications of the invention will be obvious to those skilled in theart. The appended claims in this invention generally should be construedto cover all such obvious forms and modifications which are within thetrue spirit and scope of the present invention.

1. A method for providing refrigeration to a natural gas liquefactionprocess wherein a cooling loop containing a refrigerant composition isdirected through a heat exchanger comprising controlling saidrefrigerant composition continuously by changing the liquid level in awarm end phase separator.
 2. The method as claimed in claim 1 whereinsaid refrigerant composition contains iso-pentane.
 3. The method asclaimed in claim 1 wherein the liquid level in said warm end phaseseparator decreases as the heat transfer required of said heat exchangerincreases.
 4. The method as claimed in claim 1 wherein said liquid levelis controlled on-line.
 5. The method as claimed in claim 1 wherein theliquid level in said warm end phase separator increases as the heattransfer required of said heat exchanger decreases.
 6. The method asclaimed in claim 1 wherein said heat exchanger and said warm end phaseseparator are in fluid communication.
 7. The method as claimed in claim1 wherein said cooling loop and said heat exchanger are in fluidcommunication.
 8. The method as claimed in claim 2 wherein said controlis performed by changing the amount of iso-pentane present in saidrefrigerant composition.
 9. A method for providing cooling to a processfor producing liquefied natural gas comprising the steps: a) contactinga stream of liquefied natural gas with a heat exchanger; b) contactingsaid heat exchanger with a cooling loop containing a refrigerantcomposition; c) adjusting the composition of said refrigerant bycontrolling the amount of a heavier condensable component in saidrefrigerant composition in response to performance variations in saidheat exchanger; and d) recovering liquefied natural gas.
 10. The methodas claimed in claim 9 wherein said heavier condensable component isiso-pentane.
 11. The method as claimed in claim 9 wherein saidperformance variations in said heat exchanger are selected from thegroup consisting of a temperature increase in said heat exchanger and atemperature decrease in said heat exchanger.
 12. The method as claimedin claim 9 wherein the amount of said heavier condensable component insaid refrigerant composition will increase in response to an increase intemperature in said heat exchanger.
 13. The method as claimed in claim 9wherein the amount of said heavier condensable component in saidrefrigerant composition will decrease in response to a decrease intemperature in said heat exchanger.
 14. A method for providingrefrigeration to a natural gas liquefaction process wherein a coolingloop comprising two warm end separators and a cold end separator are influid connection with each other comprising controlling the liquid levelin said warm end separators.
 15. The method as claimed in claim 14wherein said cooling loop contacts a heat exchanger.
 16. The method asclaimed in claim 14 wherein said refrigerant composition containsiso-pentane.
 17. The method as claimed in claim 14 wherein the liquidlevel in said warm end separators decreases as the heat transferrequired of said heat exchanger increases.
 18. The method as claimed inclaim 14 wherein said liquid level is controlled on-line.
 19. The methodas claimed in claim 14 wherein the liquid level in said warm endseparators increases as the heat transfer required of said heatexchanger decreases.
 20. The method as claimed in claim 16 wherein saidcontrol is performed by changing the amount of iso-pentane present insaid refrigerant composition.
 21. The method as claimed in claim 14wherein a first warm end separator is in fluid communication with asecond warm end separator.
 22. A method for providing refrigeration to anatural gas liquefaction process wherein a cooling loop comprising awarm end separator and a cold end separator are in fluid connection witheach other comprising the steps: a) controlling the liquid level in saidwarm end separator; b) removing vapor from said cold end separator; c)heat exchanging said vapor with a separate liquid nitrogen storagesystem; and d) returning the liquid from said exchange of step c) to aheat exchanger.
 23. The method as claimed in claim 22 wherein saidcontrolling of said liquid level is performed on-line.
 24. The method asclaimed in claim 22 wherein the liquid level in said warm end separatoris controlled by changing the amount of iso-pentane present in saidliquid.
 25. The method as claimed in claim 22 wherein said cooling loopcontacts a heat exchanger.
 26. The method as claimed in claim 22 whereinthe liquid level in said warm end separators decreases as the heattransfer required of said heat exchanger increases.
 27. A method forproviding refrigeration to a natural gas liquefaction process wherein acooling loop comprising a warm end separator and a cold end separatorare in fluid communication with each other comprising controlling theliquid level in said warm end separator and controlling the flow ofnatural gas from a separator to a heat exchanger.
 28. The method asclaimed in claim 27 wherein the liquid level in said warm end separatoris controlled by changing the amount of iso-pentane present in saidliquid.
 29. The method as claimed in claim 27 wherein said controllingof said liquid level is performed on-line.
 30. The method as claimed inclaim 27 wherein the flow of natural gas from a separator to a heatexchanger is controlled by the temperature of said separator where a gasstream will enter said heat exchanger and a bottom liquid stream isdirected from said separator for reentry into said separator.
 31. Themethod as claimed in claim 30 wherein said bottom liquid streamcomprises butane, propane and pentane.
 32. The method as claimed inclaim 30 wherein said gas stream from said separator after exiting saidheat exchanger is directed into said separator.